Electrochromism is a term that is used to describe the fact that certain materials change color when exposed to an electric potential. Most electrochromic materials are based on oxidation/reduction (redox) chemistry. Generally, a material is used which is one color in its oxidized form and another color in its reduced form. Based on this principle, windows and mirrors utilizing these materials can be reversibly colored or bleached, as described in U.S. Pat. No. 4,712,879. Most electrochromic displays use a redox dye, which can either be inorganic or organic. For example, a display device using inorganic oxides, such as tungsten and molybdenum oxide, as the electrochromic material is disclosed in U.S. Pat. No. 3,827,784. A display device using an organic dye (viologen) is disclosed in U.S. Pat. No. 3,806,229. These systems may suffer from a lack of flexibility in design options in that, in order to change from one color system to another, the entire chemistry of the device would need to be changed, i.e., an entirely new redox dye would need to be used. A change in the redox dye can have significant impact not only on the optical properties (color, contrast, etc.) but also on other performance characteristics, such as power requirements, response time, open circuit lifetime, etc.
U.S. Pat. No. 3,280,701 describes one possible method of making a mirror in which the optical characteristics can be varied by the use of phenylthalein or other indicator dyes to create a color change between two electrodes in a slightly acidic aqueous solution.
JP Patent 01134429 suggests a similar method for increasing response time and preventing deterioration in a film of an electrochromic material. The display device used two cells containing liquid electrolyte solution with a pH adjusted to 6.0. The cells are located in a series between two electrodes and are separated by an ion exchange resin. One of the cells contains multiple acid-base indicators having different pH regions for decoloration. Applying positive and negative voltage alternately between the electrodes enables control of pH via control of current values. Control of pH in turn controls the coloration and limits the deterioration of the electrochromic material.
Systems using indicator dyes in slightly acidic aqueous solutions, while providing more options with regard to color and optical properties than the systems using redox dyes, also have their deficiencies. Specifically, as JP 0113429 notes because these embodiments are based on redox chemistry using the decomposition of water, a voltage at least equal to the theoretical decomposition potential must be applied. Thus, the systems have relatively high power (voltage) requirements. In addition, the decomposition of water will lead to formation of oxygen and hydrogen gases at the anode and cathode, respectively. Such gas bubbles will interfere with optical properties and potentially block the electrodes from further reaction.
Possibly, the most significant complicating factor is the fact that pH systems based on the hydrolysis of water will have limited lifetimes. A change in pH over time may limit the lifetime of display devices as many electrode surfaces, including indium tin oxide, are sensitive to acidic and/or basic environments. Additionally, as two cells in a series are taught by one of the embodiments in JP0113429, manufacture is necessarily complex. Finally, the migration of ions across the barrier may also slow the response time for this system.
Despite the long recognition that color can be controlled electrochromically and despite the use for some time of electrochromism to provide privacy windows and the like, a need remains for an efficient, high-contrast electrochromic display that have a reasonably long lifetime.